This invention relates to an optical fiber connector indexable tuning tool and, more particularly to a tool for testing an calibrating tunable optical fiber connector and apparatus for tuning it.
In optical fiber communications, connectors for joining fiber segments at their ends, or for connecting optical fiber cables to active or passive devices, are an essential component of virtually any optical fiber system. The connector or connectors, in joining fiber ends, for example, has, as its primary function, the maintenance of the ends in a butting relationship such that the core of one of the fibers is axially aligned with the core of the other fiber so as to maximize light transmissions from one fiber to the other. Another goal is to minimize back reflections. Such alignment is extremely difficult to achieve, which is understandable when it is recognized that the mode field diameter of, for example, a single mode fiber is approximately nine (9) microns (0.009 mm). Good alignment (low insertion loss) of the fiber ends is a function of the alignment, the width of the gap (if any) between the fiber ends, and the surface condition of the fiber ends, all of which, in turn, are inherent in the particular connector design. The connector must also provide stability and junction protection and thus it must minimize thermal and mechanical movement effects.
In the present day state of the art, there are numerous, different, connector designs in use for achieving low insertion loss and stability. In most of these designs, a pair of ferrules (one in each connector), each containing an optical fiber end, are butted together end to end and light travels across the junction. Zero insertion loss requires that the fibers in the ferrules be exactly aligned, a condition that, given the necessity of manufacturing tolerances and cost considerations, is virtually impossible to achieve, except by fortuitous accident. As a consequence, most connectors are designed to achieve a useful, preferably predictable, degree of alignment, some misalignment being acceptable.
Alignment variations between a pair of connectors are the result of the offset of the fiber core centerline from the ferrule centerline. This offset, which generally varies from connector to connector, is known as xe2x80x9ceccentricityxe2x80x9d, and is defined as the distance between the longitudinal centroidal axis of the ferrule at the end face thereof and the centroidal axis of the optical fiber core held within the ferrule passage and is made up of three vectors. It is often the case, generally, that the ferrule passage is not concentric with the outer cylindrical surface of the ferrule (vector I), which is the reference surface. Also, the optical fiber may not be centered within the ferrule passage (vector II whose magnitude is the diametrical difference divided by two) and, also, the fiber core may not be concentric with the outer surface of the fiber (vector III). Hence eccentricity can be the result of any one or all of the foregoing. The resultant eccentricity vector has two components, magnitude and direction. Where two connectors are interconnected, rotation of one of them will, where eccentricity is present, change the relative position of the fibers, with a consequent increase or decrease in the insertion loss of the connections. Where the magnitude of the eccentricities are approximately equal the direction component is governing, and relative rotation of the connectors until alignment is achieved will produce maximum coupling.
There are numerous arrangements in the prior art for xe2x80x9ctuningxe2x80x9d a connector, generally by rotation of its ferrule, to achieve an optimum direction of its eccentricity. One such arrangement is shown in U.S. Pat. No. 5,481,634 of Anderson et al., wherein the ferrule is held within a base member which maybe rotated to any of four rotational or eccentricity angular positions. In U.S. Pat. No. 4,738,507 of Palmquist there is shown a different arrangement and method for positioning two connectors relative to each other for minimum insertion loss or maximum coupling. The arrangements of these patents are examples of the efforts to achieve optimum reliable coupling, there being numerous other arrangements and methods.
In all such arrangements for achieving optimum coupling with connectors having different magnitudes and directions of eccentricities, the tuning takes place, usually, if not always prior to the final assembly of the connector. As a consequence, an installer in the field has no control over the degree of coupling, other than by trial and error. Further, tuning of the connector cannot be performed after production of the connector is completed. Thus tuning prior to final assembly of the conductor is a step in the production process.
An optical fiber connector that can be tuned for optimum performance after the connector has been assembled would greatly decrease production costs and further, impart a greater measure of reliability to the connectors. Such a connector would be of significant commercial value.
The present invention is a tuning index tool for use in tuning tunable optical fiber connections for achieving maximum possible signal transmissivity or minimum insertion loss despite the connector being fully assembled. In a preferred embodiment of the invention the principles thereof are illustrated with a connector of the LC type for single mode fibers. It is to be understood that the principles of the invention are applicable to numerous other types of connectors such as, for example, the SC, FC and ST type connectors, as well as to other fiber optic type devices.
A connector for which the present invention is used is, for purposes of illustration, is a modified LC type connector, as shown in U.S. patent application Ser. No. 09/363,906. The basic components of such a connector as shown in that patent application comprise a ferrule-barrel assembly for holding the end of an optical fiber extending axially therethrough and a plug housing member which contains the barrel-ferrule assembly. A coil spring member contained within the housing surrounds the barrel and bears against an interior wall of the housing and an enlarged barrel member, thereby supplying forward bias to the barrel-ferrule assembly relative to the housing. The barrel member, referred to as a flange in the Anderson et al. patent, is shaped to be supported within an interior cavity within the housing in any one of four rotational orientations with respect to the central axis of the fiber holding structure. A ferrule extends axially from the enlarged barrel member and contains a fiber end therein. Thus the direction of eccentricity of the fiber relative to the central axis can have any one of four rotational or angular orientations. The connector is xe2x80x9ctunedxe2x80x9d to the extent that four orientations are possible. However, the xe2x80x9ctuningxe2x80x9d is a manufacturing step preceding final assembly of the connector, after which it is no longer xe2x80x9ctunablexe2x80x9d.
The ferrule-barrel assembly of the connector is modified so that the enlarged barrel member or flange is optimally hexagonal in shape, and has a tapered or chamfered leading surface which may be slotted. The housing is also modified so that the interior cavity is hexagonal in shape to accommodate the barrel member in any of six rotational orientations and a sloped constriction against which the leading surface bears in its forward position. Tuning of the fully assembled connector is accomplished by the application of an axial force to the barrel member, as by a spanner wrench fitted within the slots in the leading surface, sufficient to overcome the bias of the coil spring and to push the barrel portion rearwardly out of engagement with the hexagonally shaped recess in the housing and the sloped constriction. The ferrule-barrel assembly is then incrementably rotatable to any of six angular orientations, sixty degrees (60xc2x0) apart. It should be noted that a lesser number of surfaces can be used if the diagonal distance of the barrel cross-section is reduced sufficiently to allow rotation thereof within the plug housing or, alternatively if the housing bore is enlarged, which, however, weakens the walls of the housing. Fewer surfaces means larger increments of rotation and hence less precise reduction in loss. Also, more than six surfaces may be used, however, the improvement over six surfaces is slight and the clearance surrounding the barrel makes limiting rotation within the housing difficult to achieve.
In accordance with the invention, an indexable tuning tool having a spring loaded split LC adapter that is keyed and labeled to measure the optical performance of an LC connector at six different angular orientations. The tool has a longitudinal split ceramic sleeve therein for aligning two LC connector end faces. In operation, a test jumper cable having an LC connector which has an eccentricity of a magnitude greater than that of the connector to be tuned and a known direction (angular orientation), is inserted into the sleeve and the production jumper connector is fitted into the sleeve so that the connector ferrule ends abut. The opposite end of the jumper cable is connected to an optical source, or an optical detector, and the production jumper is connected to a source or detector to complete a test circuit. Ideally, the test jumper has an eccentricity of 1.8 to 3.5 xcexcm relative to the ferrule axis, and has an angular orientation of zero degrees (0xc2x0) or one hundred eighty degrees (180xc2x0), preferably the former which is an upright vertical orientation. Insertion loss measurements are then taken and the initial loss is noted. The portion of the tool holding the product jumper is spring loaded to allow separation of the fiber end faces and rotation thereof. The tool is thus rotated in sixty degree (60xc2x0) increments, with measured insertion loss being recorded at each increment. The angular orientation of the product jumper that yields minimum insertion loss is thus determined. The labeling on the tool indicates how many degrees, in sixty degree increments, the product jumper had to be rotated to produce minimum insertion loss. Inasmuch as the angular orientation of the connector of the test jumper is known, preferably, as stated hereinbefore, zero degrees (0xc2x0) or straight up or vertical, the tool indicates how many incremental stages the product jumper requires to have a corresponding vertical orientation. It is also feasible to ascertain, instead of minimum insertion loss, the angular orientation for maximum insertion loss. Rotation of 180xc2x0 from this orientation yields the orientation for minimum insertion loss. In both methods, one or the other of the extremes of insertion loss is determined.
A tool, used for tuning, in the form of a spanner wrench is included as an element of the tuning test operation. The tuning tool wrench comprises an enlarged handle shaped, such as hexagonally, for gripping from which extends a hollow sleeve having a distal end with first and second tangs extending therefrom. The sleeve is adapted and sized to fit over the ferrule of the product jumper connector with the tangs engaging the slots in the leading or front surface of the barrel member. In use, the tangs are engaged and the ferrule-barrel assembly is pushed to the rear out of engagement with the plug housing and against the spring bias, so that the ferrule-barrel assembly may be rotated the required number of degrees as indicated by the index tool to the angular orientation yielding minimum insertion loss where the connector is mated with another connector having vertical orientation of its eccentricity. The spanner wrench of the invention has a shoulder from which the sleeve extends, which butts against the housing to limit the insertion distance of the wrench. The distance from the face of the shoulder to the tangs is chosen such that the ferrule-barrel assembly, when pushed against the coil spring, does not cause the spring to bottom, which can be damaging to the spring. The barrel is then rotated to the tuned position. When the wrench is removed, the spring returns the barrel forward to its new rotated position. In this manner, the product connector is tuned. It is contemplated that all mating connections will have such a vertical orientation, hence the installer, for example, does not have to be concerned with optimum tuning.
Thus the unique structure of the connector permits tuning of the assembled connector. Further, the tuning tool enables additional rotations of the ferrule-barrel assembly when desired, for whatever reason, such as where it is desired to have operation at a predetermined value of loss, as when channel balance among several channels is desired.